1. Field of the Invention
The present invention relates to apparatus for distributed temperature sensing, and methods of using the apparatus to perform distributed temperature sensing.
2. Description of Related Art
Distributed temperature sensing (DTS) is a temperature measurement technique that uses optical fibre as a temperature sensor, and exploits Raman scattering within the fibre to determine temperature. The technique is described in Dakin, J. P. et al.: “Distributed Optical Fibre Raman Temperature Sensor using a semiconductor light source and detector”; Electronics Letters 21, (1985), pp. 569-570, and UK Patent Application GB 2140554A. A sensing fibre is deployed in an environment of interest in which temperature is to be measured. A pulse of probe light, typically a high power pulse from a laser, is launched into the fibre and propagates therein. The light undergoes scattering within the fibre from which several signals result. Rayleigh backscattering gives back-propagating light at the original probe wavelength. Raman scattering produces light at two Raman-shifted wavelengths, the Stokes and anti-Stokes signals, the amplitude of which is temperature-dependent. This scattering is generated in both the forward and backward directions. The backscattered Raman signals are detected as they emerge from the launch end of the fibre. The time between launch and detection is proportional to the distance traveled by the light in the fibre, so that the instantaneous Raman amplitude can be related to the position along the fibre of the originating scattering event. A distributed profile of temperature along the fibre is thus obtained. The anti-Stokes signal is more sensitive to temperature changes than the Stokes component, so the former is generally measured, and further improvement is often achieved by measuring both and calculating the ratio of the anti-Stokes to Stokes signals. Double-ended systems are commonly employed, in which the fibre is deployed in a loop, and measurements made from both ends of the fibre. A double-ended DTS system is described in P. di Vita, U. Rossi, “The backscattering technique: its field of applicability in fibre diagnostics and attenuation measurements”; Optical and Quantum Electronics 11 (1980), pp. 17-22. Comparison of the two measurements can be made to take account of losses in the fibre, which, unlike the temperature effects, appear opposite in sense when viewed from opposite ends of the fibre.
A DTS system typically includes filters designed to reject unwanted back-propagating light at the launch end of the fibre. This primarily relates to blocking light at the probe wavelength, to reject the Rayleigh backscattering and also Fresnel reflections of the probe pulse, while allowing the Raman signals to pass through.
Fresnel reflections may arise in the event that the optical fibre includes one or more optical connectors or couplers used to join sections of fibre together. These devices can have undesirably high reflectivity, and will reflect a portion of any forward propagating light. As mentioned, reflections of the probe pulse can be addressed with suitable filtering.
However, the forward propagating light also includes the forward Raman scattering. This signal contains the same energy as the Raman backscattering, and propagates essentially with the probe pulse. Thus, a pair of forward scattered Raman pulses (the Stokes and anti-Stokes components) builds up, with energy roughly equal to the time-integrated power of the Raman backscattered signals. The peak power of the forward Raman pulses can thus be many times the instantaneous power of the backscattered signals.
These pulses will undergo Fresnel reflection at any coupler or other reflective element along the sensing fibre, so that back-propagating components at the Stokes and anti-Stokes wavelengths are created. These will be detected along with the genuine Stokes and anti-Stokes backscattered signals, and cannot be distinguished therefrom. Thus, the detected backscattered temperature-dependent signal is disrupted at the position of the coupler, giving an inaccurate temperature measurement at that point. Further, the detector or detectors employed are typically highly sensitive to allow detection of the Raman backscattering which is of a much lower power than the original probe pulse. Fresnel reflection of the much higher power forward Raman scattering gives a high power back-propagating component, which can saturate the detectors or subsequent preamplifiers, or possibly subsequent circuitry. The instantaneous backscattered signal is lost, and also a certain amount of the subsequent backscattered signal during the time taken for the detector to recover and resume its usual operation.
The higher the reflectivity of the coupler, the greater the undesirable effects will be. For a single ended system, the effects also depend on the position of the coupler along the fibre. The degree to which the effects are problematic will depend on the accuracy desired of the temperature measurement. For some applications, an accuracy of 0.1 K is required, and Fresnel reflections will typically not allow this level of accuracy to be achieved.
Thus there is a need to address the problem of Fresnel reflections from fibre couplers and other reflective components, so that more accurate DTS temperature measurements can be made.